An implantable medical device (IMD) is implanted in a patient to monitor, among other things, electrical activity of a heart and to deliver appropriate electrical therapy, as required. Implantable medical devices include pacemakers, cardioverters, defibrillators, implantable cardioverter defibrillators (ICD), and the like. The electrical therapy produced by an IMD may include pacing pulses, cardioverting pulses, and/or defibrillator pulses to reverse arrhythmias (e.g., tachycardias and bradycardias) or to stimulate the contraction of cardiac tissue (e.g., cardiac pacing) to return the heart to its normal sinus rhythm. These pulses are referred to as stimulus or stimulation pulses.
The stimulus pulses are delivered to chambers of the heart at a stimulus output. The stimulus output represents the strength or electrical potential of the stimulus pulses. If the stimulus output of the pulses is not sufficiently large, then the stimulus pulses may be ineffective. Stimulus pulses having an output that does not exceed a stimulation threshold of the heart may not cause polarization of one or more chambers of the heart or propagate. For example, a stimulus pulse applied to an atrium at a stimulus output that is less than the stimulation threshold of the heart may not result in depolarization of the atrium, or capture in the atrium.
In order to ensure that the stimulus pulses are delivered to the heart at a sufficiently high stimulus output, the IMD may monitor and adjust the electrical potential of the stimulus pulses on a beat-by-beat basis. In one operational mode referred to as an autocapture mode, the IMD applies a stimulus pulse to a ventricle of the heart and monitors the evoked response of the ventricle. If the stimulus pulse results in capture in the ventricle, or ventricular contraction, then the IMD does not adjust the electrical potential of subsequent stimulus pulses. On the other hand, if the stimulus pulse does not result in ventricular capture, then the IMD increases the electrical potential of subsequent stimulus pulses. This monitoring of the evoked response occurs on a beat-by-beat basis in known IMDs. That is, some known IMDs may adjust the electrical potential of a stimulus pulse applied in a cardiac cycle based on the evoked response in the preceding cardiac cycle(s).
During the autocapture mode, a back-up stimulus pulse may be applied when capture is not detected. For example, if capture is not detected in a ventricle within a predetermined time period after delivering a ventricular stimulus pulse, some known IMDs supply a back-up stimulus pulse to the ventricle to ensure ventricular contraction during the current cardiac cycle. If a stimulus is delivered at approximately the same time as an intrinsic cardiac event, fusion may occur. The fusion of intrinsic and paced ventricular contractions may cause the IMD to adjust the electrical potential of the stimulus pulses too frequently. For example, fusion can cause a captured ventricular contraction to appear as a non-captured ventricular contraction caused by a ventricular stimulus pulse. The appearance of a captured contraction as a non-captured contraction may result in the IMD increasing the electrical potential of subsequent stimulus pulses, even though the current electrical potential of the pulses is sufficient to induce capture. As a result, the IMD unnecessarily wastes electrical energy of the IMD. Alternatively, a non-captured ventricular contraction caused by a ventricular stimulus pulse may appear as a captured ventricular contraction. As a result, the IMD may not increase the electrical potential of subsequent stimulus pulses, even though the current electrical potential may be insufficient to induce capture.
Additionally, some known IMDs cause a threshold search to be performed when a predetermined number of consecutive cardiac cycles exhibit a loss of capture. The threshold search involved incrementally decreasing the electrical potential of the stimulus pulses applied to the heart until a predetermined number of consecutive cardiac cycles demonstrate a loss of capture. The electrical potential of the pulses is then incrementally increased until a predetermined number of consecutive cardiac cycles exhibit capture. In situations where fusion causes captured contractions to appear as non-captured contractions, the IMD may unnecessarily initiate a threshold search. The repeated application of stimulus pulses during a threshold search can cause patient discomfort, compromise hemodynamics, and waste battery energy. Therefore, unnecessary threshold searches may result in unnecessary patient discomfort and decreased device longevity.
A need exists for an improved IMD and method of operating an IMD that enables the adjustment of the electrical potential of stimulus pulses while reducing the number of back-up stimuli and threshold searches due to the occurrence of fusion.